1. Field of the Invention
The present invention relates in general to pumps for pumping a hydraulic fluid and more particularly to oil pumps of an internal gear type, that are installed in an automotive automatic transmission.
2. Description of the Related Art
As is known, the oil pump of internal gear type (which will be called “gear pump” hereinafter) generally comprises an inner gear as a drive gear that has external teeth, an outer gear as a driven gear that has internal teeth, and a pump housing and a pump cover that are coupled to house therein both the inner and outer gears. Upon assembly, a pump chamber is defined between some of the external teeth of the inner gear and some of the internal teeth of the outer gear. The inner gear is secured at a center opening thereof to a driving shaft to be driven by the same. In operation, the inner gear is driven by the driving shaft while turning the outer gear therearound, and thus the pump chamber is subjected to a volume change thereby pumping the oil through inlet and outlet ports.
In the gear pumps as mentioned hereinabove, there is a type in which the pump cover is formed with a recess at a position that faces one side surface of the inner gear, so that there is formed an oil passage that extends from the pump cover, the recess, the center opening of the inner gear and an external part of the pump housing. In case wherein the gear pump is installed in an automotive automatic transmission, the driving shaft for driving the inner gear is formed with an axially extending oil passage for supplying a torque converter with a hydraulic or operating fluid. For ease of understanding, such oil passage will be referred to as a “torque converter pressure passage” hereinafter.
Between the pump cover and the pump housing, there is defined a pocket for housing therein both the inner and outer gears. The thickness of the pocket is somewhat greater than the face width of each tooth of each gear. Thus, between each side surface of the inner gear or outer gear and each inner surface of the pump cover or pump housing except the area where the inlet and outlet ports for the operating fluid are provided, there is defined a very thin clearance.
As is known, the operating fluid that is highly compressed by the pump is discharged to the outside of the pump through the outlet port. However, during this discharging, part of the operating fluid is forced to leak through the above-mentioned very thin clearance. The clearance between the pump cover and the inner gear is in communication with the torque converter pressure passage 8 showing the low pressure, and thus, part of the hydraulic pressure produced by the pump is led to such low pressure passage 8, which means a leak of the operating fluid. It has been revealed that the amount of the leak is proportional to the cube of the size of the clearance.
Due to provision of such clearance through which the leak of the operating fluid is caused, the pumping power of the pump is lowered, which brings about various drawbacks particularly when the pump is installed in the automatic transmission. That is, when such pump is practically installed in the transmission for feeding a hydraulic system of the transmission with a controlled hydraulic pressure, the inlet/outlet balance of the operating fluid becomes poor. That is, the amount of fluid that is to be fed to engaging elements of the transmission, such as clutches, brakes and the like, tends to show a shortage by the amount of the leak. Of course, in this case, the engaging elements fail to operate normally.
For eliminating or minimizing the leak, one method has been hitherto proposed in which the face width of each tooth of each gear is increased to minimize the thickness of the clearance that causes the leak. However, in this case, increase in weight of each gear is induced and thus the torque needed for driving the pump is inevitably increased, which causes undesired increase of fuel consumption of the engine. Furthermore, due to increase of size of the pump, the transmission having the pump installed therein fails to have a compactness.
Japanese Laid-open Patent Application (tokkai) 2003-172269 shows a gear pump that is improved in minimizing the leak. For the improvement, measures are applied to the pump for providing axial faces of each gear with a balanced hydraulic pressure. For achieving this, there are formed grooves on respective inner surfaces of the pump housing and pump cover or on the axial faces of each gear.